How did Navy Yard shooter get access to base?

With all Aaron Alexis' brushes with the law, and his possible mental health issues, how was it possible that the Navy Yard shooting suspect was allowed access to the Washington D.C. Navy Yard base?

http://archive.federaltimes.com/VideoNetwork/2679887604001/How-did-Navy-Yard-shooter-get-access-to-base-http://archive.federaltimes.com/VideoNetwork/2679887604001/How-did-Navy-Yard-shooter-get-access-to-base-http://bc_gvpc.edgesuite.net/img/963482463001/201309/498/963482463001_2679864816001_thumbnail-for-video-2679829330001.jpgHow did Navy Yard shooter get access to base?With all Aaron Alexis' brushes with the law, and his possible mental health issues, how was it possible that the Navy Yard shooting suspect was allowed access to the Washington D.C. Navy Yard base?85Shooteraaron alexisnewsbestWashingtonnavy yard shootingvpcnavyNational Newsnaval baseWUSA01:44